Resistance-coupled amplifier



Dec. 16, 1941. v J. W. CONKLlN 2,266,167

RES ISTANCE- COUPLED A MPLIFIER Filed May 7, 1938 2 Sheets-Sheet 1 8 11 fig}, Ca 12 PHASE ANGLE FREQUENCY I NV EN TOR.

A TTORNEY.

- J. .CO/VKL/N BY 7% Z I Dec. 16, 1941. N 6 2,266,167

RESISTANCE-COUPLED AMPLIFIER 4 Filed. May 7, 1938 2 Sheets-Sheet 2 DISTRIBUTED I CAPAC/MNCES 0F 54m) I6 6 {4 f /5 /F 2 E 10 EH6 2 S a m I: E w

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Patented Dec. 16, 1941 OFFICE I RESISTANCE-COUPLED AMPLIFIER James W. Conklin, Audubon, N.

Radio Corporation Delaware 1 J., assignor to of America, a corporation of Application May 7, 1938, Serial No. 206,595 3 Claims. (Cl. 179 171) This invention relates to resistance-coupled amplifiers, and more particularly to the adaptation of such amplifiers to the requirements of television and facsimile transmission.

It is an object of my invention to provide a resistance-coupled amplifier which will give substantially uniform amplification while introducing only a small phase shift which is directly proportional to the frequency.

It is a further object of my invention to provide a resistance coupled amplifier which permits of the use of multi-electrode discharge tubes such as tetrodes and pentodes so that the advantages of use of the shield grid and the suppressor grid for suppressing regeneration and oscillation is gained, together with thehigher amplification ratios which are obtainablein such tubes. Ordinarily the use of such tubes in a straight resistance-coupled circuit has been found to have its drawbacks because of the high plate resistance which tends to make them constant current output devices. Furthermore, these tubes in resistance-coupled amplifiers possess a high capacitance between the anode and ground, also between the anode and the shield grid. They are not, therefore, readily adaptable to the requirements of an amplifier which is to cover a wide range of frequencies.

Resistance-coupled amplifiers have long been used as a. means of obtaining uniform amplification over a wide range of audio frequencies. Their uniform frequency characteristic arises from the constancy of the coupling resistors over the frequency range as compared to other types of coupling, such as transformer-impedances For ordinary audio'frequencies the stray capacities and tube capacities do not alfect the performance in any noticeable degree. 5 When it is desired to amplify a wide range offrequencies, as in television, from twenty to-severalmillion cycles per second, ordinary coupling means such as transformers are impractical andresistancecoupling is one of the few practical methods. At the higher frequencies the reactances presented by the tube element capacities become comparable to the coupling resistances, reducing the effective coupling impedance which results in nonuniform amplification and objectionable unproportional phase shift.

My invention will now be described inmore detail, reference being made to the accompanying drawings, in which: A n I Figure 1 shows a conventional resistance-com pled amplifier, the characteristics-of which are hereinafter set forth in order to explain the theory of operation of myimproved amplifier,

Figs. 2 and 3 are circuit diagrams which schematically illustrate the relationships existing in a resistance-coupledamplifier, i' j p --Fig. 4'is a curve diagram showing the relationship between the ratio of interstage voltages and the angle of phase displacement which occurs at different frequencies, r

Fig. 5 is a circuit diagram of a bodiment of my invention, I

Fig. 6a is a circuit diagram of a network which is referred to me. theoretical discussion-of the problem, f

Fig. 6b is a curve diagram showingthe relatio'nship between frequency and attenuation in the network of 'Figjfia, and Fig. 7 is still another curve diagram which is referred to in explaining the theory of operation ofmyinvention. 1 Referring first to Fig. l, I show therein a typical resistance-coupled amplifier circuit where the effective tube capacitances are indicated as Ca, which is the capacitance between the anode and cathode of tube l, whileC is the capacitance between the control grid and the cathode of tube 2. For the purpose of 'clearly" explaining the interstage action two pentode tubes l and 2 are shownwhere the anode 3 of tube I is coupled to the control grid 4'of tube 2 by meansof a coupling capacitor 5. A cathode-to anode voltage is supplied to the tube by means of any suitable source 6, the positive terminal of which connects to a resistor Ra and thence to the anode 3. The same source feeds energy across the potentiometer 1. to the screen grid 8. This screen grid is maintained'atalternating current ground potential by means of a capacitor 9. 1 Input energy may be derived 'fromany suitable source coupled to the inductance I 0,' one of. the terminals-of which connectsdirectly to the control grid ll, while the other terminal of the inductance I0 is maintained at a suitable negative potential with preferred emrespect to'th'e cathode and ground by'means'i'of' a biasing source l2. Any other tem'of applying input energyto may be used, however. a 1

'1 Referring to the connections to the electrodes ofthe' tube 2, these are'for, the most part the same as are "shown with respect to tube l. and need. not, therefore, be explained in detailLxIt well known systhe control grid will be noted, however, thatthe control grid 4 is suitably'jbiased by meansof the resistor Hg in addition to the biasing "source I3, Output energy from'thetube 2 may be fed to any utilization cir cuit across the coupling condenser l4.

:In order to make clearer the improvements which are hereinafter described in more detail, it is desirable toconsider the theory of operation of the conventional circuit shown in Fig. .1. For practical purposes the effective capacitance of the coupling condenser} may be neglected be cause it has a relativelylarge value in contrast with the interelectrodecapacitances of thetubes V and these intereleotliQde' capaQitances can, there: fore, be consideredwith respe t to. theeffec ve succeeding tube.

coupling resistances such as those shown at Ra and Rg. These resistors are effectively in parallel, as will be seen upon reference to .the more simplified network diagram of Fig, 2. Here the tube has been replaced by a source E2. in circuit with a resistance Ta which may be considered as the equivalent of the plate resistance of the tube I. The control voltage fed through capacitor and applied to the grid 4 of tube 2 is denoted Eg, and its value is determined by the circuit constants of the filter in relation to the input voltage Ea. and to the interelectrode capacitance Cg. The other elements of the netanode 3 and the coupling condenser 5, however, is an impedance circuit which includes two inductances I5 and IS in series. The interconnection between these inductances is' coupled to ground by means of a capacitor 11.

The inductances l5 and I6 together with the capacitor I! represent a well known type of filter, the attenuation characteristics of which are illustrated by the curve diagram of Fig. 6b. It will be observed that frequencies up to the frequency of cut-off (fc) are passed substantially unattenuated. Beyond the cut-off frequency the Work shown in Fig. 2 have been given reference j 'terstage coupling the anode voltage of one tube must be isolated from the control grid of the Analysing the circuit of Fig. 3, it is found that with increasing frequency; as the reactance of C'a-l-C becomes comparable with the resistance R, the ratio of h E... decreases and the relative phases of the anode and grid voltages vary as indicated in Fig. 4.

This is an undesirable characteristic which may be alleviated by decreasing the coupling refor which such amplifiers are intended. Furthermorefiche phase relations between the anode and grid voltages are usually found to be erratic in different parts of the frequency spectrum.

In the amplification of television and facsimile signals it is particularly desirable to obtain a I uniform gain over the entire band with eitherno phase shift or with a phase shiftwhich is proportional to the frequency in order that the amplified wave form may be the same as the orig inal. It is permissible to have a phase shift which is proportional to the frequency as this is equivalent to an equal time displacement of all-frequencies involved and does not alter" the wave form.

Referring now to Fig. 5, gram of a resistance-coupledamplifier which'includes the improved features of my invention. Here the. troublesome capacitances are used to advantage as part of the elements of a simple pi-section low pass filter'of the so-called M-derived type. This type of filter may be made to have the desired transmission characteristics over a relatively wide band and in addition will tend to'suppress undesired high frequency disturbances. l

The circuit arrangement of Fig. 5 includes preferably a pentode discharge tube l which corresponds to the like referenced tube of Fig. 1. The input circuit for this tube is also the same as' shown in Fig. 1. Connected between. the

I'show a circuit dia-' a coupling system having the attenuation rises to infinity at f (known as the frequency. of infinite attenuation). Beyond this point the attenuation decreases. It is well known that if the constants of the filter are suitably selected for making the frequency of infinite attenuation equal approximately to 1 times the cut-off frequency, then the filter will have a nearly uniform input impedance of a pure resistance equal to the characteristic impedance up to approximately of the cut-off frequency. These characteristics are further illustrated in Fig. 7 which is drawn substantially to scale. When the ratio is equal to 1.25 it can be shown that the design factor m is equal to 0.6.

It is apparent that if the tube capacities are used as all or part of the shunt capacity elements of a filter of this type, the result will be desired character istics up to half the cut-off frequency of the resulting filter, i. e. uniform frequency response and phase shift proportional to frequency. In applying the filter circuit to the tube, it will be assumed that the band is to be as wide as possible, in which case the tube capacity and the desired coupling or load resistance will be the determinants. In the case of interstage coupling where both plate and grid capacities are involved, the larger of the two is used as the determinant and the other made equal by the use of additional shunt capacity. In general the plate-to-ground capacity will be larger. The unknown elements may, be determined from certain relations derived from standard filter formulae. The nomenclature as given below, refers to Fig. 6a, which diagrammatically depicts a filter circuit ofthe type under discussion:

C2=Largest tube or stray capacity involved Z0 Characteristic impedance= R the desired coupling resistance m =0.6 (constant selected acteristic) Then 7 I to give desired char- From the above formulae it may be seen that if the resulting cut-off frequency should 'fall below a required point (less than twice the desired transmission band) it may be increased by using a lower value of coupling resistance with a proportionate reduction in gain. In a number of practical cases the transmission band was wider than required, in which case;it could; have been reduced to take: advantage of the filter action in suppressing spurious frequencies by increasing C2 with parallel shunt capacity. In the practical cases 01 will be very small and will be found to have little eifect in the transmission band and can be omitted or accounted for in the distributed capacity of L1.

There are cases where the filter action will be very desirable, for instance, when a resistance coupled amplifier of this type is to follow a detector tube in a receiver wherein it is desired to suppress the radio frequency components of the detector output from the amplifier circuit. In this case it may be desirable to increase the number of filter sections to give increased attenuation on the undesired frequencies. The intermediate shunt capacities in the filter will then be twice the value of the terminal capacities.

The above theoretical considerations may readily be applied to the circuit diagram of Fig. 5 where a two-stage amplifier is represented. The inductances l5 and I6 will possess inherently a certain amount of distributed capacity and such shunt capacities must necessarily be taken into consideration in determining the characteristics of the filter. The value of the coupling condenser 5 is chosen as one which is suificiently large so that it will have no effect on the alternating current characteristics of the circuit over the desired transmission band.

The principle of this system of coupling is to utilize the troublesome tube and stray capacities of a resistance coupled amplifier as elements of a filter structure to give desired frequency characteristics and additional advantages such as suppression of undesired frequencies. Other filter structures than the type shown can be used where other characteristics are desired, though it is believed that the type described herein is the simplest for the general case.

It will be understood that various modifications of my invention will suggest themselves to those skilled in the art in view of the foregoing description. The scope of my invention is, therefore, defined by the claims.

I claim:

1. In a multi-stage audio-frequency amplifier system, an output circuit from a discharge tube of a first stage capacitively coupled to an input circuit for a discharge tube in a succeeding stage, a series inductance and distributed capacitance potential in both said discharge tubes, and means.

for causing attenuation in said filter action to approach an infinite value at a frequency substantially 1.25 times the cut-off frequency, said means being adapted to utilize to advantage the interelectrode capacitances of the discharge tube in the first stage.

2. A filter of the M-derived type for use as a coupling means between two electron tube stages, said filter having series-connected inductances and distributed shunt-capacitance the values of which in combination with the shunting interelectrode capacitances of the tubes in the intercoupled stages provide a substantially pure resistance coupling between said stages throughout a desired band of frequencies, means capacitively coupling the point of interconnection between said series inductances to the tube cathodes in both said stages, and means for causing said filter to have a low-pass characteristic which is substantially free from attenuation below a desired cut-off frequency and approaches infinite attenuation between said cut-off frequency and a frequency that is 25% greater.

3. A filter for use between stages of an audiofrequency amplifier, wherein each stage includes an electron discharge tube having appreciable interelectrode capacitances, said filter comprising two series inductance elements each of which possesses a certain distributed capacitance, and means for capacitively coupling the point of interconnection between said elements to points of cathodal potential in both said tubes, the inductive and capacitive values of said elements being so determined in relation to the interelectrode capacitance of the tube in the first stage as to provide a filter action of the M-derived type and also of a low pass characteristic which is equivalent to a substantially pure resistance throughout a desired band of frequencies and is substantially free from attenuation below a desired cutoff frequency, and approaches infinite attenuation between said cut-off frequency and a frequency that is 25% greater.

JAMES W, CONKLIN. 

